The present invention relates to an optical scanning apparatus, and more particularly to an optical scanning apparatus configured to scan a surface with light.
An optical scanning apparatus generally comprises a light source, a deflector and a scanning lens. The light source emits light. The deflector deflects the light emitted from the light source. The scanning lens focuses the light deflected by the deflector on a photoreceptor. Thereby, an electrostatic latent image is formed on the photoreceptor.
Such an optical scanning apparatus generally has a problem of causing a ghost image on a photoreceptor. Specifically, the scanning lens has an input surface and an output surface. The most part of light deflected by the deflector enters the scanning lens through the input surface and exits the scanning lens through the output surface as normal light. However, a part of light deflected by the deflector enters the scanning lens through the input surface and is reflected by the output surface and the input surface. Thereafter, the light exits the scanning lens through the output surface as ghost light. The ghost light is focused on a point a little shifted from the point on which the normal light is focused. Consequently, a ghost image is formed on the photoreceptor.
As an optical scanning apparatus that can prevents a ghost image from forming, for example, an optical scanning apparatus disclosed in Japanese Patent Laid-Open Publication No. 2012-88612 is known. The optical scanning apparatus includes a first scanning lens and a second scanning lens. The second scanning lens is located downstream from the first scanning lens in the light traveling direction. In the optical scanning apparatus, the first scanning lens and the second scanning lens are arranged such that ghost light generated by the first scanning lens does not enter the second scanning lens. Specifically, the first scanning lens changes the traveling direction of the ghost light generated thereby such that the ghost light passes by one side of the second scanning lens in a sub-scanning direction. In this way, the ghost light is prevented from forming a ghost image.
However, in the optical scanning apparatus disclosed in Japanese Patent Laid-Open Publication No. 2012-88612, in a case in which a plurality of light beams (for example, four light beams) are deflected, the prevention of a ghost image may not be enough. This will be described referring to FIG. 12. FIG. 12 illustrates the first scanning lens 500 and two light beams BM and BY passing through the first scanning lens 500.
In an image forming apparatus capable of forming a full-color image, an optical scanning apparatus scans a photoreceptor with four light beams BY, BM, BC and BK. In this case, the four light beams BY, BM, BC and BK deflected by a deflector pass through the first scanning lens 500 in different positions with respect to the sub-scanning direction. The following description with reference to FIG. 12 focuses on the light beams BY and BM adjacent to each other in the sub-scanning direction.
In the optical scanning apparatus disclosed in Japanese Patent Laid-Open Publication No. 2012-88612, the first scanning lens 500 inclines the ghost light of the light beam BM downward (to one side in the sub-scanning direction) from the normal light of the light beam BM. Below the light beam BM, however, the light beam BY passes. Therefore, there is a possibility that the ghost light of the light beam BM would enter a photoreceptor to be scanned with the normal light of the light beam BY. Thus, there is a possibility that the ghost light of the light beam BM would form a ghost image on the photoreceptor to be scanned with the light beam BY.